Cellular & Molecular Immunology (2013) 10, 383–392 ß 2013 CSI and USTC. All rights reserved 1672-7681/13 $32.00 www.nature.com/cmi REVIEW Non-celiac gluten sensitivity: questions still to be answered despite increasing awareness Umberto Volta, Giacomo Caio, Francesco Tovoli and Roberto De Giorgio Recently, the increasing number of patients worldwide who are sensitive to dietary gluten without evidence of celiac disease or wheat allergy has contributed to the identification of a new gluten-related syndrome defined as non-celiac gluten sensitivity. Our knowledge regarding this syndrome is still lacking, and many aspects of this syndrome remain unknown. Its pathogenesis is heterogeneous, with a recognized pivotal role for innate immunity; many other factors also contribute, including low-grade intestinal inflammation, increased intestinal barrier function and changes in the intestinal microbiota. Gluten and other wheat proteins, such as amylase trypsin inhibitors, are the primary triggers of this syndrome, but it has also been hypothesized that a diet rich in fermentable monosaccharides and polyols may elicit its functional gastrointestinal symptoms. The epidemiology of this condition is far from established; its prevalence in the general population is highly variable, ranging from 0.63% to 6%. From a clinical point of view, non-celiac gluten sensitivity is characterized by a wide array of gastrointestinal and extraintestinal symptoms that occur shortly after the ingestion of gluten and improve or disappear when gluten is withdrawn from the diet. These symptoms recur when gluten is reintroduced. Because diagnostic biomarkers have not yet been identified, a double-blind placebo-controlled gluten challenge is currently the diagnostic method with the highest accuracy. Future research is needed to generate more knowledge regarding non-celiac gluten sensitivity, a condition that has global acceptance but has only a few certainties and many unresolved issues. Cellular & Molecular Immunology (2013) 10, 383–392; doi:10.1038/cmi.2013.28; published online 12 August 2013 Keywords: celiac disease; epithelial barrier function; gut inflammation; non-celiac gluten sensitivity; wheat allergy INTRODUCTION Gluten is the main protein complex of wheat and other Growing evidence indicates that a marked increase in gluten- cereals, including barley, rye and spelt. When gluten-contain- related disorders has been observed in recent years.1,2 Many ing flours are kneaded with water, gliadins and glutenins, the factors have contributed to the development of gluten-related major components of gluten, provide viscosity and elasticity to pathology, starting with the worldwide spread of the the dough.8 These proteins are resistant to gastric digestion and Mediterranean diet, which is based on a high intake of glu- increase the permeability within the small intestine through ten-containing foods. In the Mediterranean area, the mean cytoskeletal rearrangement, overexpression of zonulin and daily gluten consumption is particularly high (approximately tight junction dysfunction.9,10 Small-intestine homeostasis is 20 g and even higher in some countries).3 Moreover, the mech- altered by gluten proteins through the inhibition of epithelial anization of farming and the growing industrial use of pesti- cell growth and the induction of apoptosis.11 cides have favored the development of new types of wheat with Gluten initiates a wide array of disorders, such as celiac a higher amount of toxic gluten peptides that cause the disease, wheat allergy and gluten-related ataxia and peripheral development of gluten-related disorders.4 In addition, bread neuropathy.2 Celiac disease is an autoimmune disorder with a and bakery products currently contain a higher quantity of well-characterized autoantigen (tissue transglutaminase). The gluten than in the past due to the reduced time of dough fer- current model of celiac disease is the result of significant mentation.5 It must also be noted that diagnostic tools for advances in our understanding of its pathogenic mechanisms. gluten-induced disorders, such as celiac disease and wheat Moreover, the availability of highly sensitive diagnostic tests allergy, have progressively improved.6,7 and more detailed histopathological criteria has completely Department of Medical and Surgical Sciences, St. Orsola-Malpighi Hospital, University of Bologna, Bologna, Italy Correspondence: Dr U Volta, Laboratory of Immunology, Department of Medical and Surgical Sciences, University of Bologna, St. Orsola-Malpighi Hospital, Via Massarenti, 9, 40138 Bologna, Italy. E-mail: [email protected] Received: 19 March 2013; Revised: 30 May 2013; Accepted: 31 May 2013 Increasing awareness of non-celiac gluten sensitivity U Volta et al 384 changed the clinical ‘scenario’ of celiac disease, allowing for the healthy food regimen and that approximately 17 million US identification of groups that are at risk for celiac disease.6 citizens (i.e., 6% of the US population) were suffering from Wheat allergy is defined as an adverse immunological reac- NCGS, although evidence-based data on NCGS prevalence in tion (IgE- and non-IgE-mediated) to gluten and other proteins the general population are not yet available.16 Global sales of contained in wheat.7 Depending on the route of allergen expo- gluten-free foods are expected to reach US$4.3 billion by sure and the underlying immunological mechanisms, wheat 2015.18 Currently, the ratio of Google to PubMed citations allergy is classified as a classic food allergy (affecting the skin, for NCGS is higher than 5000 : 1, thus raising the suspicion gastrointestinal and respiratory tract), wheat-dependent exer- that NCGS is a problem created by the media more than an 16 cise-induced anaphylaxis, occupational asthma (so-called emerging clinical entity. baker’s asthma) and rhinitis, and contact urticaria. This review discusses the current knowledge regarding The spectrum of gluten-related disorders has recently NCGS, defining its basic immunological mechanisms, patho- acquired a new member, represented by non-celiac gluten sen- genesis, clinical aspects and diagnostic criteria. Our aim is to sitivity (NCGS).1,2 Patients with NCGS test negative for celiac provide a practical appraisal of NCGS that is useful for general disease and wheat allergy, but after eating foods containing practitioners and internists in the management of this emerg- gluten, they experience symptoms that remit after gluten exclu- ing gluten-related condition. sion from the diet and recur following gluten reintroduction. BASIC IMMUNOLOGICAL MECHANISMS AND In recent years, NCGS has been regarded as an intriguing topic PATHOGENESIS by researchers. Its existence was hypothesized more than 30 years ago by a double-blind crossover trial showing that six Abnormalities of the immune system are responsible for the development of both NCGS and celiac disease.1,21 In healthy of eight women complaining of abdominal pain, bloating individuals, the immune system, through innate and adaptive and diarrhea were gluten sensitive in the absence of celiac dis- immunity, plays a central role in the maintenance of tolerance ease.12 After 20 years with no mention of NCGS, in 2000, to dietary antigens and other potential harmful pathogens, thus Kaukinen et al.13 reported that 63% of 94 adults complaining protecting the organism from the development of diseases.22 of abdominal symptoms after gluten ingestion did not satisfy The innate immune response is immediate and fast, including the diagnostic criteria for celiac disease and wheat allergy, and both cellular and humoral components.23 Several cells are because they benefited from a gluten-free diet (GFD), they were involved, including macrophages, neutrophils, dendritic cells, regarded as affected by NCGS. These two papers remained the monocytes, mast cells and natural killer T cells, expressing both only reports of the possible existence of NCGS for many years, natural killer receptors and CD1d-restricted ab-T-cell recep- and patients with symptoms after gluten ingestion but without 24 tors. Another feature of innate immunity is the humoral evidence of antitissue tranglutaminase antibodies (tTGA), secretion of complement proteins, C-reactive protein and lipo- small-intestine damage and IgE to wheat were advised to con- polysaccharide-binding protein. The adaptive immune res- tinue integrating gluten into their diet because gluten was not ponse is characterized by a delayed onset and by memory thought to be the cause of their condition. For this reason, both capacity, involving both T and B cells.25 T cells are activated gastroenterologists and allergists did not treat these patients, 14 after the interaction between major histocompatibility com- who remained in a diagnostic no-man’s land. In most cases, plex-bound peptides and T-cell receptors. CD81 T cells are these patients were regarded as suffering from mental disorders stimulated by intracellular antigens presented by major histo- and were frequently referred to psychiatrists. compatibility complex class I molecules, which are expressed in Over the past 5 years, there has been a resurgence in research all nucleated cells. The activation of CD81 cells induces apop- interest regarding NCGS, as demonstrated by the two tosis, which plays an essential role in defending the organism Consensus Conferences on NCGS held in London (2011) and against viral infections. CD41 T helper cells are activated by Munich (2012) and by several scientific contributions on this interactions